Semiconductor nanowires have become the focal point of research over the last decade due to their interesting physical, chemical and biological properties. There is particular interest surrounding silicon nanowires, as silicon is one of the most abundant materials in the earth's crust and has become a cornerstone for many of the electronic, optoelectronic, electro-chemical, and electro-mechanical devices upon which designs are based.
A silicon nanowire array on top of a silicon substrate can alter the opto-electrical properties of the bulk silicon substrate. For example, a silicon nanowire array reduces the reflection of the silicon substrate, reduces the reflection at off-angles of incidence, and increases the absorption of the silicon in ways similar to traditional pyramids or light trapping mechanisms used in solar cells.
Some of the altered optical-electrical properties of silicon nanowires compared to bulk silicon are beneficial for solar cells. However, in order to form a solar cell, the two sides of a p-n junction need to be connected to the outside world. Unfortunately, contacting nanowires is not always easy.
One device design for nanowire solar cells places vertically aligned nanowires on top of a bulk (non-nanostructured) substrate. In this design, the back contact can easily be made from the backside of the substrate. The front contact, however, is more difficult to make.
The contact resistance increases the smaller the contact area. If contacts are made on top of the nanowire array, only the tips of the wires are in contact with the metal, and hence the contact resistance may be undesirably high. Too high contact resistance adversely impacts device efficiency.
Contacting the top of the nanowires can be done uniformly with a transparent conductor. If a transparent conductor is used, some of the light is absorbed inside the conductor. In addition, the sheet resistance of transparent conductors is higher than that of metals, which leads to resistive losses. The nanowires can also be contacted by metal fingers in which case the current generated in a nanowire not directly contacted needs to travel down that wire and then back up the wire with the metal contact on top before creating current.
Nanowires may also be contacted by sputtering (or evaporating) metal on top of the wires. Although this contact method is sufficient to obtain working solar cells, the contact resistance of this method may be undesirably high for best solar cell efficiency. In addition, part of the sputtered or evaporated metal drops below the nanowires and can cause other issues that limit efficiency such as added recombination centers or shorting of the p-n junction. For example, in reference (g) cited below, sputtering of contacts on nanowire arrays approximately perpendicular to the substrate gave relatively poor efficiency (9.3%) because of resistance. The same authors later arranged the nanowires so that they were slanted, and the efficiency of the solar cell increased to 11.37%.
Some methods of making nanowire arrays allow for a contact beside the base of the nanowire array. Although this contact method is useful for processes that grow wires such as vapor liquid solid processes, it is not easy to implement for processes that etch nanowires into a bulk substrate.
Other methods of contacting a nanowire solar cell include a submerged contact, where the contact is at the base of the nanowire array, as described in U.S. Published Patent Application No. 2010/0122725. This method has the advantage that the contacts do not shade the light from the top nanowire surface. In addition, the metal silicon contact area is relatively flat compared to the nanowire array. Unfortunately, this design has the limitation that most of the incident light should be absorbed in the nanowire array before it is incident onto the submerged contact. If the light is not absorbed in the nanowires before reaching the submerged contact, much of the light will be absorbed in the metal. In situations where the wires are not long enough to absorb enough of the light to give the targeted efficiency, a submerged contact is non-ideal.
A process for fabricating nanowire arrays is described in U.S. Published Patent Application No. 2009/0256134. In this process, one deposits nanoparticles and a metal film onto the substrate in such a way that the metal is present and touches silicon where etching is desired and is blocked from touching silicon or not present elsewhere. One submerges the metallized substrate into an etchant aqueous solution comprising hydrofluoric acid (HF) and an oxidizing agent. In this way, arrays of nanowires with controlled diameter and length are produced.
When forming solar cells, the doping profile is an important consideration for optimizing the cell. A design engineer has to consider many device parameters that are affected by the doping profile, and balance conflicting requirements. One such trade-off is the surface doping; front contacts have lower resistance contact to the silicon surface if the doping of that silicon is high, e.g., greater than 1019/cm3. However, higher doping levels lead to free carrier recombination, a higher level of impurity defects, and a high surface recombination velocity; all of which hurt the efficiency of a cell. One approach to ease this trade off of doping is to have high doping under the contacts, leading to low contact resistance, and low doping between the contacts in the active region of the device, leading to higher internal quantum efficiency. This approach is referred to as a selective emitter and is often used in high efficiency solar cell designs. A selective emitter usually requires an additional patterning step, and therefore added cost to the solar cell.
Relevant information regarding silicon fabrication processes known to those of skill in the art can be found, for example, in Sami Franssila, Introduction to Microfabrication (John Wiley & Sons 2004), and the references cited there.